The Public Switched Telephone Network (PSTN) is one of the most reliable networks in the world. Reliable phone service plays an important role in the everyday life of business and residential subscribers. One contributing factor in providing PSTN reliability is redundancy.
Redundancy in this context is the ability of telecommunications equipment to continue operation without service outages in the case of failure of any component of the telecommunications equipment. To achieve a high degree of availability, telephone services can only be unavailable a few minutes a year, which is commonly referred to as 99.999% (also referred to as “five nines”) availability. A highly redundant platform minimizes these outage periods and downtime. Outages due to software, hardware and environmental conditions contribute to a significant percentage of telephone network downtime. Through fault tolerant design, telecommunication equipment vendors play a significant role in ensuring that these telephone outages are minimized.
In a carrier class redundant system, the software design leverages the hardware redundancy model to minimize the impact to system availability during outage periods and maintenance activities. Software-based recovery mechanisms are used to complement physical redundancy by minimizing the impact on subscriber traffic when a failure occurs in the primary service path. This is achieved by automating the recovery process to ensure the fastest possible failover to backup resources in order to minimize packet loss during failover. If restoration is fast enough, failure events are transparent to the subscriber.
Hardware redundancy typically involves using one or more spare devices to compensate for a failed device during normal system operation. Upon failure of a primary device, a secondary device assumes operation with no interruption in service. This combination of a primary device and a secondary device comprises the minimum set for a protection group. Redundant devices in voice gateway products include module, port, timing system and power system redundancy.
Modules (also referred to as cards) provide system control and I/O functionality in voice gateway equipment. Each module is redundant in order to guarantee carrier class redundancy. There are different redundancy techniques used depending on the type of module that is installed in the gateway.
One type of redundancy is known as 1:1 equipment redundancy and is typically used for system controller modules and broadband I/O interface modules. In a 1:1 redundancy scheme, one primary module and one secondary module comprise one protection group. That is, each primary module is matched with a secondary module. If the primary module fails, the secondary module can quickly assume traffic responsibilities because of its dynamic data synchronicity with the primary module.
Another type of redundancy is known as 1:N equipment redundancy and is used to maximize the number of in-service I/O interfaces because one secondary module can back up N primary modules in a protection group. This is especially relevant in trunking applications, as duplication of interface modules can be expensive and 1:N redundancy minimizes the cost of leasing spare lines. In 1:N configurations, the spare module cannot be fully configured because its exact configuration is not completely known until one of the primary modules in the protection group fails. At that time the secondary card is updated and takes control of the traffic for the failed module. For this reason, the switchover time for 1:N equipment redundancy is typically higher in comparison to 1:1 equipment redundancy.
Still another type of redundancy is known as Automatic Protection Switching (APS). There are two types of APS, namely 1+1 APS and 1:N APS. 1+1 APS uses one working line and one protection line. Switchover from the working line to the protection line is triggered by defects such as a loss of signal at the receiver. This means 100% redundancy, because there is a spare protection line for each working line. The second type of APS is known as 1:N APS wherein N different working lines share a spare line for backup. Economic considerations have made the 1:N APS architecture more preferred than 1+1 APS.